Well, the underdrive pulley's don't really "free up" HP so much as they help slightly with the engines ability to rev quicker due to less rotational mass. The engine still has the same horsepower it did before. The efans are sort of in the same ball park. Personally, I think the Gotts mod intake, a decent cat-back exhaust, and a Gryphon/Diablo with a custom tune is about all you can do for performance without dropping big bucks to get boosted. The 04-06 guys can benefit a bit from headers, but the 07-08 trucks have a better exhaust manifold that doesn't really hinder performance too much at stock power levels.